Typically, in computed tomography (CT) imaging systems, a rotatable gantry includes an x-ray tube, detector, data acquisition system (DAS), and other components that rotate about a patient that is positioned at the approximate rotational center of the gantry. X-rays emit from the x-ray tube, are attenuated by the patient, and are received at the detector. The detector typically includes a photodiode-scintillator array of pixelated elements that convert the attenuated x-rays into photons within the scintillator, and then to electrical signals within the photodiode. The electrical signals are digitized and then received within the DAS, processed, and the processed signals are transmitted via a slipring (from the rotational side to the stationary side) to a computer or data processor for image reconstruction, where an image is formed.
The gantry typically includes a pre-patient collimator that defines or shapes the x-ray beam emitted from the x-ray tube. X-rays passing through the patient can cause x-ray scatter to occur, which can cause image artifacts. Thus, x-ray detectors typically include an anti-scatter grid (ASG) for collimating x-rays received at the detector. Imaging data may be obtained using x-rays that are generated at a single polychromatic energy. However, some systems may obtain multi-energy images that provide additional information for generating images.
Third generation multi-slices CT scanners typically include a detector assembly having scintillator/photodiodes arrays positioned in an arc, where the focal spot is the center of the corresponding circle. The material used in these detectors generally has scintillation crystal/photodiode arrays, where the scintillation crystal absorbs x-rays and converts the absorbed energy into visible light. A photodiode is used to convert the light to an electric current. The reading is typically proportional and linear to the total energy absorbed in the scintillator.
A helical scan may be performed, in which the patient is moved while data is acquired for a number of slices. A helix mapped out by a fan beam yields projection data from which images in each slice may be reconstructed. Scanning parameters, such as x-ray tube or filament current “mA”, x-ray tube supply voltage “kV”, slice thickness, scan time, and helical pitch, can affect image quality. In addition, the x-ray tube current relates to patient x-ray dose. Typically, higher tube current improves image quality, but likewise increases patient dose. To reduce dose, x-ray tube current may be reduced, but insufficient x-ray tube current levels can result in steaking artifacts in the image.
Traditionally, to ensure sufficient image quality, an operator typically prescribes a high dose and at a fixed current level to provide a constant dose during the entire scan. However, such a dose often includes excessive x-ray flux during portions of the scan when patient attenuation is low (and thus the excess flux is unnecessary). On the other hand, if the fixed level is too low, then noise artifacts may appear in the image where the beam is highly attenuated.
Scanning algorithms have been developed as an attempt to reduce patient dose and address the aforementioned issues. For instance, one known method modulates x-ray tube current as a function of slice location. However, such an arrangement can still result in excess x-ray dose at rotational gantry angles where there is small attenuation. Another known algorithm includes modulating x-ray tube current as a function of gantry angle. However, this system may include excessive dose because attenuation characteristics of, for instance, a lung region are much smaller than a shoulder region. One known system modulates x-ray tube current as a function of both gantry angle and slice location, but in such system a modulating factor is updated during a scan and based on real photon readings.
Further, such known systems may include two scout scans such that sufficient information related to a scan may be obtained prior to actual imaging—resulting in excess dose due to the two scout scans.
Thus, there is a need to reduce patient dose while maintaining sufficient x-ray tube current to avoid streaking artifacts.